In known devices of this kind, a light source and a light receiver are provided for each field and are disposed on opposite sides of the field. A ray of light generated by the light source is directed towards the field. If the field is transparent, the ray of light meets the light receiver which generates a corresponding output signal. Otherwise there is no output signal. In this way the rack can be identified by means of a binary code.
A considerable disadvantage of the prior art device is that the absence of an output signal from a light receiver may be due not only to the scanning of an opaque field but also may be caused by a defective light source. This results in considerable uncertainty in identifying the container carrier. In many applications such uncertainty cannot be tolerated. This is particularly the case in the identification of carriers for containers for biological specimens in automatic analytical systems of the kind used in medical diagnostic laboratories.
The object of the invention, therefore, is to obviate the above disadvantage.